In an electrophotographic printer, exchanging photoconductor drums in a service or maintenance operation may give rise to an axial runout or out-of-round distortion due to working distortion or fitting error for each photographic drum exchanged, which unevenly varies the distance between the peripheral surface of the photoconductor drum and its rotary shaft center, that is its turning radius. Driving the photoconductor drum in such a state at a fixed angular speed of rotation causes the peripheral speed of the photoconductor drum to be varied or changed periodically, whereby a toner image from the photoconductor drum onto a sheet of paper is periodically deviated in transfer position. A change in peripheral speed of a photoconductor drum may also be caused by a radial runout or working error of a driving means such as gear, belt or the like for driving the photoconductor drum.
To cope with such an inconvenience, the conventional means for correcting a deviation in transfer position of an image has been that in which for each formation of the image from a photoconductor drum a registration mark is printed on a traveling body having a fixed registration mark preprinted to detect an amount of deviation in spacing between the two registration marks at all times during each image formation to effect correction of image forming position as shown in JP 2,659,191 B.
The conventional technique presents the problem that a deviation must be monitored at all times for each print image and that errors in correction due to errors in reading the moving body for correaction of the deviation are entailed at all times.
In view of the aforementioned, it is an object of the present invention to provide a method and apparatus for correcting printing images whereby a periodic deviation in image transfer position caused by working distortion and/or fitting error for each photoconductor drum can be corrected simply and less costly by only onetime measurement and correction at the time of exchanging photoconductor drums.